i) Technical Field of the Invention
This invention relates to an image reading apparatus that reads an image using an image sensor.
ii) Description of the Related Art
Conventionally, there are known image reading apparatuses that read an image from an original copy using a monochrome or color image sensor to generate monochrome or color image data. Such image reading apparatuses are incorporated, for example, in copying machines, fax machines and scanners.
These image reading apparatuses generally allow the user to input a command from an operating portion of a user interface to switch resolution modes between high and low. The low resolution mode is selected to output image data in low resolution, and high resolution mode is selected to output image data in high resolution. According to a command signal generated from the user output, the image reading apparatuses are made to operate in one of the resolution modes.
An example of conventional apparatuses having a plurality of modes is an apparatus that obtains image data in high resolution from an image sensor and generates image data in low resolution by skipping pixels which constitute the image data in high resolution.
Another example is an image reading apparatus which comprises an image sensor including a sensor having a plurality of light receiving elements in a primary scanning direction, a shift register for outputting respective pixel signals obtained from the light receiving elements arranged at even-numbered positions and another shift register for outputting respective pixel signals obtained from the light receiving elements arranged at odd-numbered positions among the light receiving elements constituting the sensor. The image reading apparatus generates image data in high resolution by using output signals from both of the shift registers when the high resolution mode is selected, and generates image data in low resolution (particularly, half of the resolution in the high resolution mode) by using output signals from one of the shift registers when the low resolution mode is selected.
However, a conventional image reading apparatus of the former example which lowers the resolution by pixel skipping involves a problem that the image reading speed of the image sensor is not improved even in the low resolution mode. Therefore, the advantage in lowering the resolution cannot be fully obtained. A conventional image reading apparatus of the latter example provided with two shift registers also involves a problem that the needs of users who demand a wide variety of choices of resolutions cannot be fully satisfied since the choices are only two, although the processing speed can be effectively improved in the low resolution mode.
The inventors of the present invention devised an image reading apparatus by providing a conventional image reading apparatus of the latter example with an additional sensor and an additional shift register (see FIG. 2).
Such an image reading apparatus is advantageous since the resolution can be switched among three resolutions by combining two sensors, and three shift registers, and the image processing speed may be improved depending on the selected resolution.
For example, when each sensor is set to read with a resolution of 600 dpi in the primary scanning direction, the image reading apparatus as above can generate image data with a resolution of 1200 dpi by using all the signals obtained from the three shift registers, generate image data with a resolution of 600 dpi by using the signals obtained from the additional shift register, and generate image data with a resolution of 300 dpi by using the signals obtained from the shift register for outputting the respective light reception signals of the light receiving elements arranged at even-numbered (or odd-numbered) positions.
In the image reading apparatus as above, however, the order of the pixel signals obtained from the image sensor does not coincide with the order of arrangement of the light receiving elements in the primary scanning direction as shown in FIG. 3, and a difference therebetween is widened as time goes by. Therefore, after sequential conversion of the pixel signals obtained from the image sensor to pixel data as digital signals, it is necessary to sort the pixel data in their original order. Number with brackets shown in FIG. 3 represent light receiving positions of light receiving elements in the primary scanning direction which correspond to pixel signals.
In other words, if the sorting operation is performed after all the pixel data are stored in a memory once, areas for both storing all the pixel data before and after the sorting are necessary. Therefore, the amount of memory required is increased. Furthermore, if such a technique is adopted, access times to the memory during the sorting operation are increased, and thus, the processing speed of the overall image reading apparatus is lowered.